1. Field of the Invention
The invention relates to a method and apparatus for performing color space conversion, more particularly to a method and apparatus with reduced look-up tables for converting digitized luminance-chrominance color space signals to digitized RGB color space signals.
2. Description of the Related Art
It is desirable to merge a video signal with graphic signals in a multi-media computer system. The video signal may come from a television image processing system having a capture or frame grabbing capability, or from a compressed video playback of a CD-ROM or network transmission. Color space conversion is needed in image processing applications to convert luminance-chrominance color space signals, which offer the advantages of a lower transmission bandwidth and a lower data storage requirement, into RGB color space signals, which are used when displaying an image on a computer monitor.
CCIR 601, which was proposed by the Comite Consultatif International des Radiocommunications (CCIR), establishes the following formulas for converting from the YCbCr luminance-chrominance color space to the RGB color space:
______________________________________ R = Y + 1.402(Cr-128) (a.1) G = Y - 0.714(Cr-128) - 0.344(Cb-128) (a.2) B = Y + 1.772(Cb-128) (a.3) ______________________________________
If U and V are used to represent the shifted chrominance components (Cb-128) and (Cr-128), respectively, Equations (a.1) to (a.3) can be rewritten as follows:
______________________________________ R = Y + 1.402V (b.1) G = Y - 0.714V - 0.344U (b.2) B = Y + 1.772U (b.3) ______________________________________
where Y ranges between [0, 255], and U and V range between [-128, 1271] in an 8-bit representation for each of the Y, Cb and Cr color space components.
Color space conversion is often implemented by employing multipliers or look-up tables to achieve the matrix multiplication operations. Look-up tables are preferred because of their less complicated constructions. It is noted that the matrix multiplication operations dominate the hardware complexity of a color space converting apparatus. As such, the number of look-up tables is critical in determining the cost of implementing the color space converting apparatus. To implement the YCbCr to RGB color space conversion of Equations (a.1) to (a.3), a conventional color space converter usually requires four look-up tables to perform the matrix multiplication of chrominance components. Although the use of four look-up tables is less expensive to implement as compared to another conventional color space converter which uses a 3-by-3 multiplication matrix, a further reduction in the number of look-up tables is desirable.
In a co-pending U.S. patent application, entitled "Method And Apparatus Requiring Fewer Number of Look-Up Tables For Converting Luminance Chrominance Color Space Signals To RGB Color Space Signals," and filed on Jun. 10, 1997 by the Applicant, it has been proposed that, by linearly combining the conversion formulas, equations (b.1) to (b.3) can be rearranged as follows to result in RGB color combination signals:
______________________________________ R - G = 0.714(2V) + 0.344(U + 2V) (c.1) B - G = 0.714(2U + V) + 0.344(2U) (c.2) R + B - G = Y + 0.714(2U + 2V) + 0.344(2U + 2V) (c.3) B + G = 2Y + 0.714(2U - V) (c.4) R + G = 2Y + 0.344(2V - U) (c.5) ______________________________________
Equations (c.1) to (c.5) list a set of possible linear combinations of equations (b.1) to (b.3). Consequently, as Equations (c.1) to (c.5) use only two coefficients, namely 0.714 and 0.344, for matrix multiplications, no more than two look-up tables may be used to convert luminance-chrominance color space signals to RGB color combination signals. Therefore, conversion from the luminance-chrominance color space to the RGB color space can be implemented using fewer than four look-up tables by converting the luminance-chrominance color space signals to the RGB color combination signals expressed as a function of predetermined linear combinations of the chrominance color space signals as defined by the appropriate conversion formulas, and by linearly combining the resulting RGB color combination signals to obtain the RGB color space signals.
The apparatus disclosed in the aforementioned U.S. patent application comprises: a first combining unit for generating a plurality of linear combinations of the chrominance color space signals and at least one binary combination of the luminance color space signal; a multiplexed multiplication unit connected to the first combining unit to receive the linear combinations of the chrominance color space signals therefrom, the multiplexed multiplication unit including no more than two look-up tables which contain digitized transformation values for performing matrix multiplications of the linear combinations of the chrominance color space signals; a second combining unit connected to the multiplexed multiplication unit and the first combining unit, the second combining unit linearly combining the digitized transformation values outputted by the multiplexed multiplication unit and the binary combination of the luminance color space signal to generate three RGB color combination signals; and a third combining unit connected to the second combining unit, the third combining unit linearly combining the RGB color combination signals to obtain the RGB color space signals.
While the apparatus of the aforementioned U.S. patent application permits conversion of digitized luminance-chrominance color space signals to digitized RGB color space signals using less than four look-up tables, it is desirable to further reduce the sizes of the look-up tables used therein so that a cost-effective hardware implementation can be achieved.